The original standard of rad alt had 2 range settings, high (0-5000 ft) and low (0-500 ft). On the control panel the pilot could set the desired range and the limit lights according to whatever SOP was in force. When above the setting, the amber light was on, when at the limit the green light was on and when below the limit the red light was on. The nav had no idea what limit the pilot had set and so if you were flying with a 'twitchy' nav, especially on a check ride, a suitable limit could be set such that the red light never came on - or so I have been told .....
This rad alt worked perfectly well below 50 ft although it did overread when more than about 30 deg of bank was applied. The overwater SOP was to set the baro alt to read the ame as the rad alt and then you had an extra height reference when at extreme bank angles.
These rad alts would interfer with each other when in close formation if on the same range setting and thus the night/IMC low level SOP was, from memory, for the leader to set HIGH with the limit lights at 500 ft and for the wingmen to set LOW with the limit lights at 400 ft. The later rad alts did not suffer from this interference but I had left the squadron when they came in so I am not sure what the SOP became, although I did fly the trial and can vouch for their functionality.